At 8:26 AM -0700 4/30/08, Henry Baker wrote:

Lemesee...

How come our solar system isn't a centrifuge? Does the solar system order the planets in terms of density?

If not, what makes a centrifuge different from the solar system? I.e., at what densities of planetary matter would things start working differently?

The solar system doesn't have a wall. (?) Paul

OK, here's a stab at an answer. The solar system has a large attracting central force. This force is balanced, if you will, by the planets' orbiting. The planets would, in the absence of the sun, go flying off in straight lines; the gravity yanks the momentum vectors around--the velocity (momentum) vector changes, even though the speed is roughly constant, so there is an acceleration (change in the momentum vector) to match the central force. A centrifuge is a rapidly spinning system, where there is, essentially, no attracting central force. The reason that the test tubes don't go flying off in straight lines is that they are held (pushed towards the center) by the supporting apparatus--basically, they are pushed at the bottom of the tubes with a force that counteracts the tendency to go flying off. This is much like a gravitational field where the test tube is sitting on the lab bench--there is a normal force pushing up at the bottom of the test tube that keeps the material from falling through the tube. This is different than the planetary case, where the planets are in free fall (note: free fall != no acceleration). If you wish, gravitational forces cause a free fall because they accelerate any apparatus around a body the same as the body itself--the apparatus exerts no extra force on the body: they fall together. If I whirl a bucket of water around me, the bucket pushes on the water, which feels the force of the bucket (or the bottom of the test tube, for a centrifuge) pushing on it. My arm is pulling on the bucket directly, but not the water; the water is being pushed inward by the bucket. Gravity (if they were in orbit) would pull on them both directly, and the water would not feel any force due to the bucket. Bill C. -----Original Message----- From: math-fun-bounces@mailman.xmission.com [mailto:math-fun-bounces@mailman.xmission.com] On Behalf Of Henry Baker Sent: Wednesday, April 30, 2008 9:26 AM To: math-fun Cc: Dan Asimov Subject: Re: [math-fun] And I suppose heavy objects fall faster than light ones? Lemesee... How come our solar system isn't a centrifuge? Does the solar system order the planets in terms of density? If not, what makes a centrifuge different from the solar system? I.e., at what densities of planetary matter would things start working differently? At 07:24 PM 4/29/2008, Mike Stay wrote:

It separates things by density, not mass.

On Tue, Apr 29, 2008 at 6:50 PM, Dan Asimov <dasimov@earthlink.net> wrote:

...

Mike Speciner wrote:

<< [quote of passage from NY Times article that Mike evidently considers erroneous] >>

Mike, I don't recall if I ever understood just how centrifuges work. Would you be so kind as to explain?

--Dan

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